The present invention is directed to a process for saturating paper, particularly paper which is to be used for the manufacture of masking tape and label stock where superior wet strength, edge tear and delamination resistance are required.
Nonwoven fabrics ("nonwovens") usually contain substantial amounts of long synthetic fibers which are bonded using chemical, mechanical or thermal techniques and which generally contain little or no hydrogen bonding. In contrast, paper is generally comprised substantially of shorter cellulose fibers which are hydrogen bonded using conventional paper manufacturing techniques.
In practice coatings are then applied as post-treatments to the already formed paper sheets or nonwovens for a variety of purposes, i.e., to strengthen them or apply a functional coating so as to make them waterproof or greaseproof, or adhesive, or to size them, to make them glossy. Many of these treatments are mutually exclusive and each has its own particular problems. Thus, a pigmented coating composition which, for example, is used to provide a glossy coating such as found on paper used for magazines has completely different requirements than does a saturant type binder which is used to impregnate or saturate the paper web thereby giving the paper integrity.
More particularly, a saturant is used to impart a combination of tensile strength and stretch to the paper sheet, a property often referred to as "toughness". Other desirable properties which a saturant provides to the paper sheet include wet strength, folding endurance, flexibility, internal tear, edge tear, delamination resistance and resistance to physical degradation and discoloration due to heat and light aging. While the addition of certain comonomers, including N-methylol containing monomers, has been suggested in order to improve the strength properties of the saturants, the use of these crosslinking agents has been found to detract from other properties such as edge tear and fold endurance. These saturants of the prior art, therefore, fail to provide the required balance of properties for use in stringent applications such as in the case of papers which are to be used as base stock in the manufacture of masking tape, book cover stock, and label stock. As a consequence, styrene butadiene rubber based latices are generally used for these industrial applications although these latices are deficient in the areas of color, light and ultraviolet stability.